1. Field of the Invention
The invention concerns a reconfigurable zonal beam forming system for an antenna on a satellite in orbit.
The invention also concerns a method of optimizing reconfiguration by a system of the above kind.
2. Description of the Prior Art
Recent years have seen very substantial growth in new markets for transmission by satellites, in many applications. This growth results in particular from "Global Information Infrastructure" and "Global Mobile Communication" systems, a phenomenon that has been accentuated by the relaxing of national laws governing telecommunications in some countries, to the point of "total deregulation" in some cases. The number of operators offering varied services has increased rapidly and users have been offered a more comprehensive choice and greater flexibility, enabling an optimum fit of the resources offered to their requirements.
It is therefore becoming necessary to make projections for the very long term future, given that there is a period of 2 to 3 years between the signing of a contract for the launch of a telecommunication satellite and the arrival of the satellite in orbit, and given that the service life of a satellite of this kind is typically 12 to 15 years on average.
To obtain a sufficient return on investment, this requires the onboard telecommunication system to be very flexible. In practical terms, this means that it must be possible for the coverage of the satellite to be modified in situ, i.e. in orbit.
Considering the far field radiation pattern of the antenna, i.e. the imprint of the beam on the surface of the Earth, the beam can be characterized by a number of parameters: directivity, transmitted power distribution (curve in two dimensions), etc. It must be possible to modify all of these parameters if the beam is to be reconfigurable at will.
Conventional satellites transmitting in the C, Ka and Ku bands are conventionally subject to various constraints which reduce numerous aspects of flexibility: beam coverage, frequency plan, maximum power per channel, etc.
Attempts have naturally been made to develop devices and/or methods of circumventing these limitations, but only a modest improvement in flexibility has been achieved, usually at the cost of a substantial increase in the amount and/or complexity of satellite hardware.
Of the methods proposed in the prior art, the most interesting ones appear to be of the type using distributed amplification within the beam forming section of the onboard transmission system, which is the system feeding the antenna of the satellite.
Without seeking to be exhaustive, two types of prior art solution may be cited.
The first is described in patent U.S. Pat. No. 5,115,248 (Antoine Roederer). This discloses a multibeam antenna feed device. According to this patent, a number of contiguous narrow beams ("spots") are generated. The problem to be solved is that of producing simultaneously good overlap between adjacent beams and effective illumination of a reflector forming the transmit antenna.
To achieve this, overlapping source elements are used to generate the aforementioned adjacent beams. To be more precise, the source elements is controlled by a separate matrix of amplifiers in each overlap region. By interleaving the outputs of the matrices to feed the locations systematically, a continuous array of beams can be generated.
However, although this method definitely enables reconfiguration of the transmitted radiation, it has a number of disadvantages. In particular, as just indicated, it necessitates the use of multiple matrices, which greatly increases the cost and the complexity of the hardware. Moreover, this approach cannot be generalized to the reconfiguration of complex zonal beams unless a very high degree of imbalance between the amplifiers is acceptable.
Another example is described in an article by Howard H. S. LUH: "A Variable Power Dual Mode Network for Reconfigurable Shaped Beam Antenna", published in "IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION", Vol. AP-32, N.sup.0 12, Dec. 1984.
To generate variable phase excitation and amplitude at the feed ports, variable phase-shifter devices must be inserted between the hybrid circuits to modify the power distribution at the hybrid output ports. Further phase-shifter devices are used to adjust the relative phases of the channels (see FIG. 2: a, b and c of the above document).
The complexity of the array increases as the number of feeds requiring variable excitation increases. As a general rule, systems like this have been developed to employ centralized power concepts. Increasing the number of phase-shifters in the channels increases the losses and makes it difficult to generate simultaneous narrow beams.
As a general rule, a prior art system using distributed amplification in the beam forming section is of the form shown diagrammatically in FIG. 1 accompanying this description.
The input power I is divided into signals of equal amplitude by a splitter SP followed by variable phase-shifters .PHI..sub.1 through .PHI..sub.4 (assuming four channels to make the example more concrete). The outputs of the phase-shifters .PHI..sub.1 through .PHI..sub.4 are transmitted to the inputs of four amplifiers Amp.sub.1 through Amp.sub.4 the outputs of which drive a fixed coupling network FCN based on one more Butler matrices. Finally, the outputs of the FCN supply the power for excitation of source elements S.sub.1 through S.sub.4 of the antenna.
It is therefore entirely possible to use this system, which constitutes a Butler matrix-based first level beam forming network, to generate and supply an amplitude distribution to the source elements S.sub.1 through S.sub.4.
This result is definitely obtained if phase-shifted signals of equal amplitude are transmitted to the inputs of the FCN. However, a problem arises on attempting to reconfigure the beam to obtain a different beam with an arbitrary contour.
To obtain generalized and flexible reconfiguration of zonal beams it is necessary to provide for the excitation of each source to be variable, not only in amplitude but also in phase, and to comply very strictly with optimal values.
When prior art circuits like that shown in FIG. 1 are used, the input phase-shifters .PHI..sub.1 through .PHI..sub.4 can be adjusted to obtain a satisfactory phase match for the required new arbitrary contour to be obtained. However, it is not generally possible simultaneously to obtain an adequate distribution of amplitudes at the source elements and because of this the result of reconfiguration is significant deterioration of the new zonal beam (compared to optimal excitation of the source elements).
On the other hand, the phase-shifters can be adjusted to generate a reasonable distribution of amplitudes between the source elements compared to a required optimum (or target value), but it is then generally not possible to obtain a good balance of the phase distribution. This also results in significant deterioration of the zonal beam.
The aim of the invention is to overcome the disadvantages of the prior art devices whilst retaining a beam forming system of the above type.
It procures, simultaneously, good distribution in phase and in amplitude and therefore optimum excitation of the source elements of the antenna. It therefore provides a reconfigurable zonal beam made up of narrow beams that are fixed or variable.
To achieve this, in accordance with a first important feature of the invention, further second level phase-shifters are inserted between the first level forming network RC (see FIG. 1) and the antenna sources S.sub.1 through S.sub.4 (more generally, S.sub.x where x is any integer).
A reasonably balanced but not entirely exact amplitude distribution can be obtained by adjusting the input phase-shifters. By also adjusting the output (second level) phase-shifters it is possible to obtain simultaneously the best possible distribution of phases for the source elements.
This feature alone products a better result than that obtained by the prior art systems.
Nevertheless, there remain two basic problems:
1/ As just indicated, the amplitude balance of the signals is not perfect, resulting in a small but real deterioration of the reconfigured beam;
2/ There is generally no exact solution to this problem and many input signal phase combinations produce results that are similar in practise, in particular by allowing for all possible combinations of interconnections between the feed ports of the source elements and the outputs of the coupling network.
An exhaustive search method can of course be used, but analysis shows that for high order matrices, typically of order 8 or above, very long computation times become necessary, even if very powerful and very fast computers are used, typically one month or more. Moreover, there is no guarantee that the best global solution will be obtained.
Accordingly, in accordance with a second important feature of the invention, provisions are implemented to optimize the beam reconfiguration method of the invention and to obtain fast convergence towards a global solution.
To achieve this, the fixed input splitter supplying equal power amplitude signals is replaced by a substitute coupling network. This input network is identical to the pre-existing "normal" network, except that the Butler matrix is the inverse matrix of the latter network. This makes it possible to find immediately an exact solution to the source element amplitude distribution. One property of the Butler matrix type transponder principle is exploited. A signal transmitted to a particular input of a Butler matrix will be transmitted to a single output in a manner that can be predicted. By the principle of superposition, the relative amplitude distribution of the signals at the output ports, and therefore at the source elements, can be imposed at the input ports.
However, this arrangement generally leads to a significant imbalance between the amplifiers. A minimal imbalance can be obtained by rotating the relative phases of the input signals. At the end of this process, the error in the excitation of the source elements is nulled and the aforementioned minimum imbalance obtained. By this method all possible combinations of source clement interconnections can be rapidly evaluated, in contrast to the prior art. By defining error functions as explained hereinafter and by launching a gradient search algorithm, the lowest global error (amplifier imbalance and feed excitation error) can be determined. In a final phase of the process, the substitute network is withdrawn. The attenuators are adjusted in accordance with the values determined in the previous phases.